1. Field of the Invention
The present invention relates to a developer agitating method and a developer agitating apparatus which are employed for image forming apparatuses such as electro-photographic copying machines, printers, etc.
2. Description of the Prior Arts
Multi-color electrophotographic copying machines are provided with a plurality of developer units. Therefore, in order to prevent the whole body of the copying machine from increasing in size, it is required to make each developer unit compact.
However, in trying to design a compact developer unit by use of a method where a developer consisting of toner and carrier is circulated in a rotation direction of a sleeve similar to a standard copying machine, since a circulation direction of the developer is extremely small, supplied toner is not sufficiently mixed with the developer and, consequently, is not sufficiently charged, so that the toner scatters.
Conventionally, a method has been employed, for compact developer units, where an agitation direction is increased, as shown in FIG. 1, by circulating the developer by use of a spirals 10a and 10b on an agitating passage 45 along a partition 15 in a direction along a development sleeve 20. The arrows in FIG. 1 show the direction of the flow of the developer. Also, a T/D (wherein T represents an amount of toner and D represents an amount of developer) of the developer is sensed by a toner density sensor 30 consisting of a permeability sensor provided at the bottom of the agitating passage 45 to control an amount of toner supplied through a toner supply mouth 60.
When a spiral is used for agitating and carrying developer as described above, the amount of developer which is provided in a developer unit is an important factor for controlling the T/D of the developer.
That is, when a tip of a fan of the spiral 10a is sufficiently exposed out of the developer, the developer is pushed toward a proceeding direction while crumbling like an avalanche between the fans against a rotation direction, which is the best condition for the agitation by a spiral.
However, when the T/D is sensed by the toner density sensor 30 which is provided at the bottom of the agitating passage 45 and that employs a permeability of the carrier to control an amount of toner, the following inconvenience occurs.
For example, when a toner consuming area of an original is extremely large, the amount of the developer temporarily decreases, so that the spiral 10a is over-exposed out of the developer. As a result, the carrier included in the developer hardly contacts the toner density sensor 30 provided at the bottom of the agitating passage 45 because of an increase of air included in the developer, so that a sensing voltage decreases to stop the toner supply. This is because the toner density sensor 30 senses a carrier density in the developer from a permeability of the developer to obtain the T/D.
Moreover, when the toner density temporarily exceeds a reference value so that a fan 12 of the spiral 10a is nearly covered with the developer such as when the original to be copied is changed from an original having a large toner consumption area to an original having a small area, the developer on the surface is carried without mixed with the supplied toner. As a result, the supplied toner is carried with the developer on the surface of the developer and is never sensed by the toner density sensor 30 provided at the bottom of the agitating passage 45. Therefore, the toner supply never stops, so that toner is over-supplied to further increase the amount of the developer. Thus, a vicious cycle arises where the fan 12 of the spiral 10a is covered with the developer and the further supplied toner is never sensed by the toner density sensor 30 so that the toner scatters out of the developer unit.